Some models of SPLat have an RTS (Request To Send) output. Normally this will be manipulated automatically as part of a communications function. However, for testing is is handy to be able to manipulate it programmatically.
The instruction
SPxCmd0 1,!CPU
will turn on the RTS output (if it exists)
The instruction
SPxCmd0 0,!CPU
will turn off the RTS output (if it exists)